Glossary28 terms, plainly defined
The email infrastructure glossary
What SPF, DKIM, DMARC, bounces, suppressions and the rest actually mean — and, where it matters, how Sendix handles them.
- BIMI
- Brand Indicators for Message Identification: a DNS TXT record (default._bimi) pointing to an SVG logo that supporting mailbox providers can show next to authenticated mail. It requires DMARC enforcement, and some providers also require a verified mark certificate.
- In SendixWhen you add a domain, Sendix checks your website for an SVG logo and only offers a default._bimi record if it finds one.
- Bounce (hard and soft)
- A message the receiving server refused. A hard bounce is permanent — the address or domain doesn’t exist — and the address should not be mailed again. A soft bounce is temporary, such as a full mailbox or a busy server, and is retried.
- In SendixPermanent failures emit email.bounced and add the address to the suppression list; temporary failures emit email.delivery_delayed while Sendix retries.
- Webhooks
- Click tracking
- Rewriting links in an email through a redirect host so each click can be recorded before the reader reaches the destination.
- In SendixClicks emit email.clicked. Customers can use their own tracking subdomain (a CNAME to track.sendix.dev) so links stay on their brand.
- Complaint (spam report)
- A recipient marking a message as spam. Mailbox providers report complaints through feedback loops; a high complaint rate damages sender reputation quickly.
- In SendixComplaints emit email.complained and suppress the address with the reason complained.
- Deliverability
- The likelihood that a message reaches the inbox rather than the spam folder or a rejection. It depends on authentication (SPF, DKIM, DMARC), sender reputation, list quality, content and complaint rates.
- Deliverability guide
- DKIM
- DomainKeys Identified Mail: the sending server signs each message with a private key, and receivers check the signature against a public key published in DNS. A valid DKIM signature proves the message wasn’t altered and was authorised by the signing domain.
- In SendixSendix generates a DKIM key per domain and asks you to publish its public key as a TXT record at <selector>._domainkey.<your-domain>; key rotation emits domain.dkim_rotation_started.
- DKIM selector
- The label that tells receivers which public key to fetch: selector._domainkey.example.com. Different providers use different selectors, so several can sign mail for the same domain at once.
- DMARC
- Domain-based Message Authentication, Reporting and Conformance: a TXT record at _dmarc.<domain> that tells receivers what to do with mail that fails SPF and DKIM alignment (none, quarantine or reject) and where to send aggregate reports.
- In SendixThe suggested record is v=DMARC1; p=quarantine with aggregate reports sent to Sendix. A domain can have only one DMARC record — keep your existing one if it already has a policy.
- DMARC alignment
- The requirement that the domain in the visible From header matches the domain authenticated by SPF (the Return-Path) or by DKIM (the d= signing domain). Relaxed alignment accepts subdomains of the same organisational domain.
- Double opt-in
- Confirming a subscription with a second step — the subscriber clicks a link in a confirmation email — before any marketing mail is sent. It proves consent and keeps typos and fake addresses off lists.
- In SendixAudience contacts can be sent a confirmation email; the confirmation click records the consent proof.
- Audience
- Idempotency key
- A unique value sent with an API request so that repeating the request — after a timeout or a job retry — has the same effect as sending it once.
- In SendixSend an Idempotency-Key header or idempotency_key field (8–128 characters); a repeat returns the original email id with "idempotent": true.
- Inbound email
- Receiving email in an application rather than a mailbox: the domain’s MX records point to a service that parses each message and hands it to your code.
- In SendixReceiving-enabled domains deliver messages as parsed data through the email.received webhook and the /v1/emails/receiving API.
- Inbound email parsing
- IP warm-up
- Gradually increasing the volume sent from a new IP address so mailbox providers can build a reputation for it. Sending full volume from a cold IP often leads to throttling or spam-folder placement.
- In SendixSendix tracks the warm-up state of its sending IPs and ramps new ones gradually.
- List-Unsubscribe
- An email header that lets mailbox providers show an unsubscribe button. With List-Unsubscribe-Post (RFC 8058) the unsubscribe happens in one click without opening a web page; large providers require it for bulk senders.
- In SendixSendix hosts unsubscribe links (GET and POST /unsubscribe/{token}) and records unsubscribes as suppressions.
- Marketing email
- Promotional or editorial mail sent to a list — newsletters, announcements, campaigns. It requires consent and an easy unsubscribe, and is best sent separately from transactional mail.
- In SendixSendix sends audience mail through Broadcasts with the bulk priority lane.
- Broadcasts
- MTA-STS
- SMTP MTA Strict Transport Security: a policy published over HTTPS (plus a _mta-sts TXT record) that tells sending servers to require TLS with a valid certificate when delivering to a domain’s MX hosts.
- In Sendixsendix.dev publishes an MTA-STS policy in enforce mode.
- MX record
- The DNS record that names the servers accepting mail for a domain, with priorities. Changing MX records moves where that domain’s incoming mail goes.
- Open tracking
- Recording when a message is opened, usually through a tiny image loaded from a tracking host. Privacy features in some mail apps preload images, so opens are a directional signal rather than an exact count.
- In SendixOpens emit email.opened when tracking is enabled for the domain.
- Return-Path (envelope sender)
- The address bounces are sent to, set in the SMTP MAIL FROM command. SPF is checked against its domain, so a custom Return-Path on your own domain is needed for SPF alignment.
- In SendixSendix uses bounce.<your-domain>, a CNAME to bounce.sendix.dev, so bounces are processed automatically and SPF aligns with your domain.
- Sender reputation
- How mailbox providers score a sending domain and IP based on history: complaint and bounce rates, spam-trap hits, authentication and engagement. It largely decides inbox versus spam placement.
- SMTP
- Simple Mail Transfer Protocol, the protocol servers use to send email to each other. Applications usually hand mail to a provider over SMTP submission on port 587.
- In SendixSendix accepts SMTP submission at smtp.sendix.dev:587 with STARTTLS, using your API key as the password.
- SMTP
- SPF
- Sender Policy Framework: a TXT record listing the servers allowed to send mail for a domain’s Return-Path. A domain may have only one SPF record, and evaluating it may take at most 10 DNS lookups.
- In SendixSendix asks you to add include:mail.sendix.dev to your SPF record.
- STARTTLS
- An SMTP command that upgrades a plain connection to an encrypted TLS connection before any credentials or message content are sent.
- Suppression list
- Addresses a sender must not mail — because they hard-bounced, complained, unsubscribed or were added manually. Honouring it protects reputation and respects recipients.
- In SendixManage it with /v1/suppressions (reasons: hard_bounce, soft_bounce, complained, unsubscribed, manual, invalid); sends to suppressed addresses emit email.suppressed.
- TLS-RPT
- SMTP TLS Reporting: a _smtp._tls TXT record that asks sending servers to report TLS negotiation failures when delivering to your domain — the monitoring companion to MTA-STS.
- Transactional email
- Email triggered by a user’s action or account state — sign-up verification, password resets, login codes, receipts, invoices, security alerts. It is expected, time-sensitive and sent one recipient at a time.
- Use cases
- Webhook
- An HTTP request a service sends to your endpoint when something happens — for email, events such as delivered, bounced, opened or received.
- In SendixSendix posts signed JSON events, retries failed deliveries, and lets you inspect and replay deliveries.
- Webhooks
- Webhook signature
- A cryptographic signature over the webhook body that lets the receiver confirm the request came from the provider and wasn’t modified or replayed.
- In SendixX-Sendix-Signature carries t=<unix>,v1=<hex>; compute HMAC-SHA256(secret, t + "." + raw body), compare in constant time and reject timestamps older than five minutes.
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